1. Field of the Invention
The invention relates to a magnetic resonance apparatus, comprising a magnet system for generating a steady magnetic field in a measuring space, a gradient coil system for generating gradient fields in the measuring space, a power supply source for the gradient coils, which comprises at least a gradient signal generator and a number of gradient amplifiers, each of which is connected between an output of the gradient signal generator and at least one of the gradient coils, and also a noise cancellation device for generating a sound signal which is, in at least one region which is referred to as a region of silence, in phase opposition with a sound signal generated by the gradient coils in the region of silence.
2. Description of the Related Art
An apparatus of this kind is known, for example from the Abstract in English of JP-A-1-291 848. The gradient coils, situated within the magnetic field of the magnet system, act as loudspeaker coils when current flows therethrough, so that in the measuring space a sound is generated which may be very annoying to a patient being examined in the measuring space. The noise cancellation device serves to generate a sound signal which is in phase opposition with said sound so that the sound intensity in the measuring space is strongly reduced. In the known apparatus the sound caused by the gradient coils is detected, after which a sound signal of opposite phase is formed from the detected sound signal so as to be applied to a sound reproducer in the measuring space. This method is based on the assumption that the sound measured at a given instant T is substantially identical to the sound to be compensated at a later instant T+.DELTA.T. This has inter alia the following drawbacks:
1. some time elapses before the noise cancellation has reached its ultimate value; PA1 2. when the gradient coils are driven in a non-periodic manner (for example, in the case of quickly changing preparatory gradients, a non-linear profile sequence, changing slice orientations), the noise cancellation device cannot follow the sound signals caused by the gradient coils, so that the noise cancellation is absent or very incomplete; PA1 3. the switch-on and switch-off behaviour of the gradient coils is clearly audible, notably in the event of triggered exposures.